JP2020164994A - Steel member nitriding treatment method - Google Patents

Abstract

To provide a steel member nitriding treatment method capable of substantially increasing a γ' phase fraction in a short treatment time.SOLUTION: A steel member nitriding treatment method comprises: a first-stage nitriding treatment step of nitriding a steel member in a nitriding gas atmosphere at 550-610°C with a nitriding potential at which ε-phase or ε-phase and γ'-phase nitride layers are formed; and following the first-stage nitriding treatment step, a second-stage nitriding treatment step of nitriding the steel member while holding the steel member in a nitriding gas atmosphere at 480-520°C for 10-60 minutes. In a method of nitriding a steel member by performing a first-stage nitriding treatment step of forming ε-phase or ε-phase and γ'-phase nitride layers and a second-stage nitriding treatment step of increasing the γ'-phase fraction of the nitride layer, the γ'-phase fraction can be substantially increased by carrying out the second-stage nitriding treatment step in a short time.SELECTED DRAWING: Figure 1

Description

The present invention relates to a method for nitriding a steel member that forms a nitride compound layer on the surface of the steel member by nitriding.

Steel members such as gears used in transmissions for automobiles are required to have high pitching resistance and bending fatigue strength, and in order to meet these requirements, carburizing treatment and other methods for strengthening steel members such as gears are required. The strength has been increased by nitriding. In particular, it has been clarified that a component having a nitride compound layer mainly composed of the γ'phase has high pitching resistance and bending fatigue strength, and various manufacturing methods thereof have been proposed.

For example, in Patent Document 1, after nitriding with a high nitriding potential, nitriding with a low nitriding potential is performed (both nitriding is carried out at a temperature of 520 to 610 ° C.) to precipitate a γ'phase. The processing method is disclosed. Further, in Patent Document 2, a steel member is subjected to a nitriding treatment step at 600 ° C., then cooled, and subsequently, it takes 5 minutes or more in an atmosphere in which the iron nitride compound phase at 425 ° C. to 600 ° C. does not grow. It is disclosed that 40% or more of the γ'phase is precipitated by carrying out a passing step. In Patent Document 3, the steel member is subjected to nitriding treatment at 550 to 650 ° C. in the first nitriding zone and nitriding treatment at 400 to 550 ° C. in the second nitriding zone, and the second It is disclosed that in nitriding, the γ'phase is precipitated because it is exposed to an atmospheric gas having a temperature lower than that of the first nitriding. In Example 8 of Patent Document 3, the second nitriding is carried out under the conditions of 500 ° C. and 90 minutes.

WO2015 / 046593A1 Japanese Unexamined Patent Publication No. 2016-194111 JP-A-2018-59195

In the step of forming the nitrided compound layer in the nitriding treatment, it is necessary to perform the treatment at a high temperature of 600 ° C. from the viewpoint of the formation rate (productivity) of the nitrided compound layer and the sufficient formation of the final γ'phase. In Patent Document 1 and Patent Document 3, a nitride compound layer is formed by high-temperature nitriding treatment, and then nitriding in the second stage is performed to precipitate the γ'phase. It takes about 60 to 120 minutes for the second stage nitriding (in Patent Document 3, it takes 90 minutes when the temperature is 500 ° C.).

From the viewpoint of productivity of the nitrided steel member, it is desired to precipitate more γ'phases in a short time treatment. Therefore, in the present invention, a steel that performs a first-stage nitriding treatment step of forming ε-phase or ε-phase and γ'phase nitriding compound layers and a second-stage nitriding treatment step of increasing the γ'phase fraction of the nitride compound layer. An object of the present invention is to provide a method for nitriding a steel member, which can greatly increase the γ'phase fraction in a short second-stage nitriding treatment.

In order to solve the above problems, according to the present invention, a steel member is nitrided in a nitriding gas atmosphere at 550 to 610 ° C. having a nitriding potential at which ε-phase or ε-phase and γ'phase nitriding compound layers are formed. Following the first-stage nitriding treatment step and the first-stage nitriding treatment step, the nitrided steel member is held at 480 to 520 ° C. for 10 to 60 minutes in a nitriding gas atmosphere to perform nitriding treatment. Provided is a method for nitriding a steel member, which comprises a step nitriding process.

In this nitriding treatment method, the second-stage nitriding treatment step may be carried out in a nitriding gas atmosphere having a nitriding potential of 0.2 to 2.0 atm ^−0.5 . Further, the nitriding potential in the first-stage nitriding treatment step may be 0.2 to 2.0 atm ^−0.5 . Further, after performing the second-stage nitriding treatment step, a cooling step of cooling the steel member to room temperature may be further provided. Further, in the second-stage nitriding treatment step, the nitriding steel member may be held at 480 to 520 ° C. for 10 to 45 minutes. Further, in the second stage nitriding treatment step, the nitriding steel member may be held at 490 to 510 ° C. for 10 to 45 minutes.

Further, according to the present invention, a first-stage nitriding treatment in which a steel member is nitrided in a nitriding gas atmosphere at 550 to 610 ° C. at a nitriding potential at which ε-phase or ε-phase and γ'phase nitriding compound layers are formed. Following the step and the first-stage nitriding treatment step, the nitriding-treated steel member is nitrided under the condition that the γ'phase fraction of the nitrided compound layer is increased by 30% or more within 60 minutes. A method for nitriding a steel member having a processing step is provided.

According to the present invention, the first-stage nitriding treatment step of forming the ε-phase or ε-phase and γ'phase nitriding compound layers and the second-stage nitriding treatment step of increasing the γ'phase fraction of the nitrided compound layer are performed. In the nitriding treatment method for steel members, it is possible to greatly increase the γ'phase fraction by carrying out the second-stage nitriding treatment step in a short time.

It is explanatory drawing which shows the example of the structure of the heat treatment apparatus. It is a figure which shows the profile of the temperature and the nitriding potential KN of one Embodiment of the nitriding treatment method of this invention. It is a graph which shows the distribution in the depth direction of N concentration and C concentration in the work surface layer of Comparative Example 1. This is a Phase MAP showing the distribution of the γ'phase and the ε phase, which was created by EBSD analysis of the work surface layer of Comparative Example 1. 6 is a graph showing the distribution of N concentration and C concentration in the depth direction on the work surface layer of Example 1. This is a Phase MAP showing the distribution of the γ'phase and the ε phase, which was prepared by EBSD analysis of the work surface layer of Example 1. It is an iron-nitrogen-carbon system phase diagram.

Hereinafter, the technical idea and specific embodiments of the present invention will be described with reference to the drawings.

In the present invention, a nitriding compound layer containing the γ'phase as a main component is formed on the surface of the steel member (base material) by gas nitriding the steel member.

The nitriding treatment applied to the steel member as the object to be treated is performed using, for example, the heat treatment apparatus 1 as shown in FIG. As shown in FIG. 1, the heat treatment apparatus 1 includes a carry-in portion 10, a heating chamber 11 for carrying out the first-stage nitriding treatment step, a heating chamber 12, a cooling chamber 13, and a carry-out conveyor 14 for carrying out the second-stage nitriding treatment step. Have. In the case 20 placed in the carry-in portion 10, a steel member made of a carbon steel material for machine structure or an alloy steel material for machine structure such as a gear used for an automatic transmission is housed. An entrance hood 22 having a door 21 that can be opened and closed is attached to the entrance side (left side in FIG. 1) of the heating chamber 11.

<1st stage nitriding process>
In the first-stage nitriding treatment step in the nitriding treatment method for a steel member of the present invention, the steel member is subjected to a nitriding gas at 550 to 610 ° C. having a nitriding potential at which ε-phase or ε-phase and γ'phase nitriding compound layers are formed. Nitriding is performed in the atmosphere. The steel member contains Fe (iron) as a main component and usually contains 0.02 to 0.8% by mass of carbon, and the presence of this carbon is necessary for the formation of the ε phase. The "main component" means that the content of Fe in the steel member is 90% by mass or more, preferably 94% by mass or more, and more preferably 96% by mass or more. Means that. In addition, steel members can be used for various purposes or as unavoidable impurities, such as Si (silicon), Mn (manganese), P (phosphorus), S (sulfur), Ni (nickel), Cr (chromium), Cu. It may contain elements such as (copper) and Mo (molybdenum). The content of each of these steel members is approximately 0.05 to 2% by mass for Si, Mn and Mo, approximately 1% by mass or less for Ni, Cu and Cr, and approximately 0.05% by mass for P and S. It is said to be as follows.

Before the nitriding treatment, pre-cleaning may be performed to remove dirt and oil from the material to be treated (steel member). As the pre-cleaning, for example, vacuum cleaning in which oil or the like is dissolved in a hydrocarbon-based cleaning liquid and degreased and dried by evaporating, alkaline cleaning in which degreasing treatment is performed with an alkaline cleaning liquid, or the like is preferable.

The nitriding potential (KN) is by the ratio of the partial pressure P of the NH ₃ gas constituting the gas nitriding atmosphere (NH ₃₎ with _{H 2} partial pressure of the gas P _{(H 2),} a well-known formula (1) expressed.
KN = P (NH ₃ ) / P (H ₂ ) ^3/2 ... (1)

The nitriding potential KN in the first-stage nitriding treatment step is preferably 0.2 to 2.0 atm ^−0.5 because a ε-phase or ε-phase and γ'phase nitriding compound layer is formed on the steel member (surface). It is more preferably 0.4 to 1.0 atm ^−0.5 .

The temperature of the nitriding gas atmosphere in the first-stage nitriding treatment step is 550 ° C to 610 ° C. If it is lower than 550 ° C., the formation rate of the nitride compound layer becomes slow, the carbon concentration of the ε phase in the compound layer increases, and it becomes difficult to obtain the γ'phase in the second nitriding treatment step. If the temperature of the nitriding gas atmosphere is higher than 610 ° C., softening of the steel member, increase in strain, and the like may occur. From the viewpoint of the formation rate of the nitrided compound layer and the prevention of softening of the steel member, the temperature of the nitride gas atmosphere is preferably 570 to 600 ° C.

The nitriding gas atmosphere is composed of N ₂ gas, NH ₃ gas and H ₂ gas, and the respective proportions are adjusted so as to achieve a predetermined nitriding potential KN.

Further, the nitriding time of the first-stage nitriding treatment step may be controlled so that a nitriding compound layer having a target thickness is formed. Generally, the nitriding time is in the range of 1 to 8 hours.

A specific embodiment of the first-stage nitriding process described above will be described with reference to the heat treatment apparatus 1 of FIG. A heater 25 is provided in the heating chamber 11. A nitride gas composed of N ₂ gas, NH ₃ gas, and H ₂ gas is introduced into the heating chamber 11, and the nitride gas is heated to a predetermined temperature (550 to 610 ° C. as described above) by the heater 25. The first-stage nitriding treatment of the steel member carried into the heating chamber 11 is performed. A fan 26 is mounted on the ceiling of the heating chamber 11 to agitate the nitriding gas in the heating chamber 11 to make the heating temperature of the steel member uniform. An openable and closable intermediate door 27 is attached to the outlet side (right side in FIG. 1) of the heating chamber 11.

In the heat treatment apparatus 1, the case 20 in which the steel member is housed is carried into the heating chamber 11 from the carry-in portion 10 by a pusher or the like. Prior to this delivery, the pre-cleaning described above may be performed, if necessary. Further, in the heating chamber 11, a nitride gas is introduced before the case 20 is carried in, and the gas is heated to a predetermined temperature (550 to 610 ° C. as described above) by the heater 25. At this time, the fan 26 is rotated at, for example, 1000 rpm so that the inside of the heating chamber 11 is heated evenly, and the nitride gas is agitated.

Then, after the case 20 containing the pre-cleaned steel member is carried into the heating chamber 11 as needed, the nitriding treatment of the steel member carried into the heating chamber 11 while stirring the nitride gas with the fan 26. Is done. The steel member is at room temperature when it is carried into the heating chamber 11, and nitriding does not occur in that state. Further, since the steel member is at room temperature, generally, when it is put in the heating chamber 11, the temperature in the heating chamber 11 is lowered. This is heated to the originally set temperature by the heater 25. Along with the heating, the temperature of the steel member itself rises to a high temperature similar to the temperature inside the heating chamber 11, and nitriding starts.

In this first-stage nitriding treatment step, the nitriding potential KN in the heating chamber 11 is controlled within a specific range. Specifically, for example, NH respect ₃ partial pressure of the gas P (NH ₃₎ and _{H 2} gas partial pressure P _{(H 2),} the NH ₃ gas atmosphere in the heating chamber 11 by the infrared gas analyzer, _{H 2} It can be controlled by automatically adjusting the total amount and flow rate ratio of the nitrided gas supplied to the heating chamber 11 while analyzing the gas online with a thermal conductivity type gas analyzer. By controlling the partial pressure of these gases, the nitriding potential KN is controlled to be 0.2 to 2.0 atm ^−0.5 . The nitriding potential KN may be changed in the middle of the first-stage nitriding treatment step.

<Second stage nitriding process>
Following the first-stage nitriding treatment step, a second-stage nitriding treatment step for increasing the γ'phase fraction on the surface of the nitride compound layer is performed. In the second-stage nitriding treatment step, the temperature of the steel member nitrided in the first-stage nitriding treatment step is maintained at 480 to 520 ° C. for 10 to 60 minutes in a nitride gas atmosphere. By holding the steel member at such a specific temperature, the γ'phase fraction in the nitride compound layer formed in the first-stage nitriding treatment step can be greatly increased in a short time.

Although it is not clear why the γ'phase fraction can be greatly increased in a short time by keeping the temperature at a specific temperature, the present inventor originally contains nitrogen that penetrates into the steel member from the nitride gas atmosphere as well as the steel member itself. Considering that the behavior of carbon is important, it is considered as follows. The ε phase generated in the region of 550 ° C. or higher, which is the implementation temperature of the first-stage nitriding treatment step, is placed in the region of 480 ° C. to 520 ° C. in the second-stage nitriding treatment step to dissolve carbon. It is considered that the phase is separated into two phases, an ε phase having a higher carbon and a γ'phase having a lower carbon concentration. FIG. 7 is an iron-nitrogen-carbon phase diagram when the nitriding potential KN is constant at 0.5, calculated by THERMO-CALC of THERMO-CALC. What can be read from FIG. 7 is that the stable region of the ε phase shifts to the high carbon concentration side when the temperature drops, and as shown in FIG. 7, the ε phase near the phase boundary at 600 ° C. If so, when the temperature of this ε phase is lowered to 500 ° C, it is expected that the phase will be separated into the ε phase with a higher carbon concentration and the γ'phase with a lower carbon concentration. It turns out that movement is necessary. Although the agreement between the calculated value and the measured value is still insufficient, it is the temperature that the ε phase generated at 550 ° C or higher decreases by holding it at 480 to 520 ° C, and the γ'phase increases instead. Due to the change in the carbon solid solution amount of the ε phase due to the change, it is necessary to hold for 10 to 60 minutes in order to increase the γ'phase fraction, which is the time required for carbon transfer for phase separation. It is believed that there is.

In the second stage nitriding process, if the steel member is held at a temperature lower than 480 ° C., the progress rate of phase separation decreases, and if it is held for a long time, the γ'phase fraction can be increased, but γ in a short time. 'The phase fraction cannot be increased. On the other hand, when the temperature at which the steel member is held is higher than 520 ° C., the ε phase is stable, phase separation is unlikely to occur, and the γ'phase fraction cannot be increased in a short time. From the viewpoint of greatly increasing the γ'phase fraction in a short time, it is preferable to hold the steel member at 490 to 510 ° C.

Regarding the time for holding the nitrided steel member at the predetermined temperature described above, the phase separation greatly progresses in 10 minutes, and there is almost no change thereafter. Since the second-stage nitriding process is a short-time process, the upper limit of the holding time is 60 minutes. From the above, the holding time is 10 to 60 minutes. From the viewpoint of greatly increasing the γ'phase fraction in a short time, the holding time is preferably 10 to 45 minutes.

The nitriding potential KN of the nitriding gas atmosphere in the second-stage nitriding treatment step is preferably 0.2 to 2.0 atm ^−0.5 so that the α phase is not generated. How high the γ'phase fraction is by holding the steel member at 480 to 520 ° C. in a nitriding gas atmosphere is determined by the nitriding potential KN, and depends on the γ'phase fraction required for the steel member. To adjust the nitriding potential KN. From the viewpoint of increasing the γ'phase fraction, the nitriding potential KN is more preferably 0.25 to 0.55, and particularly preferably 0.25 to 0.45.

A specific embodiment of the second-stage nitriding process described above will be described with reference to the heat treatment apparatus 1 of FIG. For example, by adjusting the total amount and flow rate ratio of the nitriding gas introduced into the heating chamber 12, the nitriding potential KN is controlled to be 0.2 to 2.0 atm ^−0.5 so that the α phase is not generated. (Note that the nitriding potential KN may be changed in the middle of the second-stage nitriding treatment step). Then, the inside of the heating chamber 12 is heated by the heater 28 to keep the temperature at 480 to 520 ° C. The steel member that has undergone the first-stage nitriding treatment in the heating chamber 11 is carried into the heating chamber 12. The steel member has the same temperature (550 to 610 ° C.) as in the heating chamber 11, and gradually lowers to the range of 480 to 520 ° C. in the heating chamber 12 at a lower temperature, and is held at that temperature for 20 minutes, for example. ..

During the nitriding treatment, the fan 29 in the heating chamber 12 is rotated at, for example, 1000 rpm to uniformly diffuse the nitriding treatment gas.

In the above, the mode in which the first-stage and second-stage nitriding treatments are performed by the heat treatment apparatus having two heating chambers has been described. However, by adjusting the temperature in one heating chamber, the first-stage and second-stage nitriding treatments are performed. The step nitriding treatment may be carried out sequentially.

<Cooling process>
When the second stage nitriding process is completed, a cooling process is performed. In the cooling step, the steel member subjected to the second stage nitriding treatment is cooled to room temperature. Since this step cools the steel member having a high temperature of about 500 ° C., it is preferable to carry out the step in a dedicated cooling chamber (closed space). Further, the cooling step is preferably carried out in an atmosphere of an inert gas such as nitrogen gas (air cooling) to prevent oxidation.

As a method of carrying out the cooling step, cooling oil may be prepared and a steel member may be immersed in the oil for rapid cooling at a cooling rate higher than that of air cooling. If the phase separation into the ε phase and the γ'phase has not progressed sufficiently in the second-stage nitriding treatment, the phase separation also proceeds during cooling (that is, the γ'phase fraction increases). Can be considered. When a plurality of steel members are nitrided at one time, there is a slight difference in the cooling history received by the steel members in the cooling step, and therefore, the γ'fraction of the steel members after cooling may vary. In such a case, it is conceivable that the phase separation does not substantially proceed by quenching using oil. However, in the present invention, in the second-stage nitriding process, the steel member is held for a sufficient time in the temperature range in which the phase separation is preferably advanced, so that the phase separation of the steel member is substantially completed. There is no need to perform such quenching.

A specific embodiment of the cooling step described above will be described with reference to the heat treatment apparatus 1 of FIG. For example, the case 20 in which the steel member that has undergone the second-stage nitriding treatment step is housed is carried from the heating chamber 12 into the cooling chamber 13. The inside of the cooling chamber 13 has a nitrogen gas atmosphere at room temperature, in which the steel members are cooled to room temperature.

FIG. 2 shows the profile of the temperature and the nitriding potential KN of one embodiment of the nitriding treatment method for steel members of the present invention described above. For example, before charging the steel member, the temperature inside the heating chamber 11 is raised to about 600 ° C., and the nitriding potential KN is set to 0.4 atm ^−0.5 . The case 20 in which the steel member at room temperature is housed is carried into the heating chamber 11 and held in a nitride gas atmosphere at 600 ° C. for 2 hours. The steel member is heated in the heating chamber 11 to raise the temperature, and when the temperature rises, nitriding starts.

The steel member subjected to the first-stage nitriding treatment is carried into a heating chamber 12 having a chamber temperature of 500 ° C. and a nitriding potential KN of 0.4 atm ^−0.5 . The steel member is cooled to the room temperature in the heating chamber 12 (the temperature is lowered in about 2 minutes) and held at that temperature for 18 minutes (a total of 20 minutes, the steel member is held in the heating chamber 12). ..

Subsequently, the steel member is carried from the heating chamber 12 into the cooling chamber 13. The inside of the cooling chamber 13 has a nitrogen gas atmosphere at room temperature. Here the steel member is cooled to room temperature.

By performing the nitriding treatment under the conditions of the nitriding treatment method for the steel member of the present invention described above, it is possible to obtain a nitriding steel member having a nitride compound layer containing the γ'phase as a main component on the surface. The steel member thus obtained is strengthened by forming a nitrogen diffusion layer and a nitride inside, and a γ'phase-rich nitride compound layer is formed on the surface to provide sufficient pitching resistance and bending fatigue strength. Have.

According to the present invention, a first-stage nitriding process for forming ε-phase or ε-phase and γ'phase nitriding compound layers and a second-stage nitriding process for increasing the γ'phase fraction of the nitride compound layer are performed. In the nitriding treatment method, in particular, the second-stage nitriding treatment step can be performed in a short time.

From such an effect, the present invention
"The first-stage nitriding process of nitriding a steel member in a nitriding gas atmosphere at 550 to 610 ° C. and the first-stage nitriding process having a nitriding potential at which ε-phase or ε-phase and γ'phase nitriding compound layers are formed. Following the nitriding treatment step, a two-stage nitriding treatment is performed on the nitrided steel member under the condition that the γ'phase fraction of the nitrided compound layer is increased by 30% or more (preferably 35 to 70%) within 60 minutes. A method for nitriding a steel member, which comprises a nitriding process. "
You can also catch it.

Further, as compared with the carburizing or carburizing nitriding treatment, the nitriding treatment of the present invention is a treatment at an austenite transformation temperature or lower, so that the amount of strain is small. Further, since the quenching step, which is an essential step in the carburizing / carburizing nitriding treatment, can be omitted, the amount of strain variation is small. As a result, a nitrided steel member having high strength and low strain can be obtained.

Although Patent Document 3 is a continuous processing furnace, such a continuous processing furnace will continue to operate under the same conditions. In this case, if there are multiple types of steel members to be nitrided (meaning multiple types in terms of the composition and structure of the steel members themselves and the required γ'phase fraction), let's process them in a continuous processing furnace. Then, the following process is required. First, the processing of the steel member, which has been continuously processed under certain conditions, is completed and taken out from the furnace. Next, the steel members to be processed under different conditions are carried into the processing furnace. Then, the nitriding process is performed by changing the conditions from now on (like a batch process). It is very inefficient to carry out such a process in a continuous processing furnace, and there is a problem in terms of cost.

If the nitriding treatment method of the present invention is operated in a batch manner, when there are a plurality of types of steel members, the conditions can be changed according to each type. Therefore, according to the present invention, high-mix low-volume production can be suitably carried out at an appropriate cost.

Further, the nitriding potential KN is a partial pressure ratio of ammonia and hydrogen, and in order to reduce KN, it is necessary to use a large amount of expensive hydrogen. For example, Patent Document 1 sets the KN of the second-stage nitriding treatment to 0.16 to 0.25, but in the present invention, the γ'phase fraction can be greatly increased in a short time without reducing the KN to that extent. , It is advantageous in terms of cost.

Although preferred embodiments of the present invention have been described above, the present invention is not limited to such examples. It is clear that a person skilled in the art can come up with various modifications or modifications within the scope of the technical idea described in the claims, and of course, the technical scope of the present invention also includes them. It is understood that it belongs to.

[Comparative Example 1]
Nitriding was performed using a plate-shaped test piece (30 mm × 40 mm, thickness 5 mm) which is a steel member (steel type A) as a work (workpiece). In the first-stage nitriding treatment step, the nitriding treatment was carried out by holding for 2 hours in a nitriding gas atmosphere at 600 ° C. having a nitriding potential KN of 0.4 atm ^−0.5 .

Subsequently, the work was cooled to room temperature in the cooling chamber (the second stage nitriding treatment was not performed). Since one work was put in the cooling chamber for cooling, the cooling rate in this case was very fast, and the work temperature became 400 ° C. or less in 2 minutes from the start of cooling. The same applies to the subsequent comparative examples and examples.

The alloy composition of steel type A is as shown in Table 1 below.

Further, the above nitriding treatment and the subsequent nitriding treatments of Comparative Examples 2 to 11 and Examples 1 to 4 were carried out in a furnace having an internal volume of about 300 L. The gas composition according to each nitriding potential KN is shown in Table 2 below.

[Comparative Example 2]
A work similar to that used in Comparative Example 1 was subjected to the first-stage nitriding process under the same conditions as in Comparative Example 1.
Subsequently, the work was placed in a nitriding gas atmosphere at 550 ° C. having a nitriding potential KN of 0.4 atm ^−0.5 for 5 minutes, and the work temperature was lowered to 550 ° C.
Next, the work was rapidly cooled to room temperature in a cooling chamber.

[Comparative Examples 3 to 11, Examples 1 to 4]
The same workpieces used in Comparative Example 1 were subjected to the first-stage nitriding process under the same conditions as in Comparative Example 1, followed by rapid cooling (without the second-stage nitriding process), or various types shown in Table 3 below. The second stage nitriding process and the cooling process were carried out in the temperature history. The temperature of the nitriding gas atmosphere in the first-stage nitriding process, the nitriding potential KN, the time (holding time) in the first-stage nitriding process, the holding temperature T of the steel member in the second-stage nitriding process, and the temperature of the steel member are maintained. The time required to lower the temperature to the temperature T, the holding time at the holding temperature T, the nitriding potential KN, the time during which the steel member is in the temperature range of 480 to 520 ° C., and the cooling method are shown in Table 3 below.

Comparative Example 3 will be described as an example of how to read Table 3. In Comparative Example 3, the first-stage nitriding treatment step was carried out under the same conditions as in Comparative Example 1. Subsequently, as the second-stage nitriding treatment step, the work is placed in a nitriding gas atmosphere at 550 ° C. having a nitriding potential KN of 0.4 atm- ^0.5 for 5 minutes, the work temperature is lowered to 550 ° C., and further 550 ° C. It was held for 5 minutes. Then, as a cooling step, the same quenching as in Example 1 was carried out.

<Measurement of γ'phase fraction>
The γ'phase fractions of the nitrided workpieces obtained in Examples 1 to 4 and Comparative Examples 1 to 11 were determined by EBSP analysis. For the analysis, an EBSP (Electron Backscatter Diffraction Pattern) apparatus mounted on an FE-SEM (model: JSM7001F manufactured by JEOL) was used. In the EBSP method, the Kikuchi pattern generated by electron backscatter diffraction when an electron beam is irradiated to a sample with a large inclination of about 70 ° in the SEM sample chamber is projected on a fluorescent screen and captured by a TV camera or the like, and the index of the pattern is further measured. This is a method of measuring the crystal orientation of the irradiation point. A plate-shaped test piece mirror-polished with a diamond (particle size 1 μm) buff and further polished with colloidal silica abrasive grains (particle size 0.05 μm) was used for analysis. Using analysis software (OIM Analysis), a Phase MAP was created in which the phases were separated based on the crystal structure considered in advance and the obtained pattern (Comparative Example 1 is shown in FIG. 4, and Example 1 is shown). (Shown in FIG. 6), the fractions of each phase of ε and γ'in the nitride compound layer were analyzed.

The above results and nitriding treatment conditions are shown in Table 3 below.

Comparative Examples 1 to 7 and Examples 1 and 2 have the same nitriding conditions in the first-stage nitriding process. In Comparative Example 1, the work is rapidly cooled after the first-stage nitriding process, and the work temperature is 400 ° C. or lower 2 minutes after the start of cooling. Since the formation of the γ'phase does not substantially occur in the temperature range of 400 ° C. or lower, the γ'phase fraction of the work obtained in Comparative Example 1 is immediately after the first-stage nitriding treatment step in the examples after Comparative Example 2. It can be said that it is the γ'phase fraction of the work. From the results in Table 3, the γ'phase fraction was increased by 35% or more by the second-stage nitriding treatment step under the nitriding treatment conditions of Examples 1 and 2. The same can be said for Comparative Examples 8 and 9, Example 3, Comparative Examples 10 and 11, and Example 4. As described above, by carrying out the second-stage nitriding treatment step at about 500 ° C. after the first-stage nitriding treatment step at a high temperature of 600 ° C., a large amount of γ'phase can be generated in a short time.

FIG. 3 shows the distribution of N concentration and C concentration in the depth direction by the electron probe microanalyzer (EPMA) on the work surface layer of Comparative Example 1, and FIG. 5 shows the N concentration and C concentration by EPMA on the work surface layer of Example 1. The distribution in the depth direction of the concentration is shown.

3 and 4 and 5 and 6 show the case of quenching to room temperature after the first-stage nitriding process (Comparative Example 1) and the case of holding at around 500 ° C. for a predetermined time (second-stage nitriding process) (Example). It shows the difference in the γ'phase fraction of 1). Of the nitride compound layer, the portion that looks relatively light (white) is the γ'phase, and the portion that looks relatively dark (black) is the ε phase. In Comparative Example 1, the carbon concentration in the ε phase is low and the distribution is gentle (almost the same amount of carbon is detected up to a depth of about 0.00 to 0.015 mm). On the other hand, in Example 1, it is shown that the carbon concentration in the ε phase is increased and the carbon is concentrated (the carbon concentration is decreased to a depth of about 0.01 mm, and the depth is 0. The carbon concentration has increased up to about 015 mm). It is considered that carbon is transferred in the nitrided compound layer by holding the mixture at around 500 ° C. (second nitriding treatment step), and phase separation from the ε phase to the γ'phase + ε phase occurs.

[Comparative Examples 12 to 14, Examples 5 to 7]
Various workpieces are subjected to the first-stage nitriding process under the conditions shown in Table 4 below, followed by quenching (without the second-stage nitriding process) or the second-stage nitriding process with various temperature histories shown in Table 4 below. And a cooling step was carried out. Regarding the first-stage nitriding process, the nitriding potential KN for the first hour of the whole (2 hours) is 2.0 atm ^−0.5, and the nitriding potential KN for the latter 1 hour is 0.4 atm ^{−0. It was set to 5.5} . Steel type used in each Example and Comparative Example, temperature of nitriding gas atmosphere in first-stage nitriding process, nitriding potential KN, time (holding time) in first-stage nitriding process, steel member in second-stage nitriding process The holding temperature T, the time required for the temperature of the steel member to cool down to the holding temperature T, the holding time at the holding temperature T, the nitriding potential KN, the time during which the steel member is in the temperature range of 480 to 520 ° C., and the cooling method. It is shown in Table 4 below.

The alloy compositions of the steel types used in each of the Examples and Comparative Examples shown in Table 4 are as shown in Table 5 below.

The nitriding treatments in Comparative Examples 12 to 15 and Examples 5 to 7 were performed in a furnace having an internal volume of about 5000 L, and the gas composition according to each nitriding potential KN is as shown in Table 6 below.

The present invention is useful as a nitriding technique for steel members.

1 Heat treatment device 10 Carry-in part 11 Heating chamber 12 Heating chamber 13 Cooling chamber 14 Carry-out conveyor 20 Case 21 Door 22 Entrance hood 25 Heater 26 Fan 27 Door 28 Heater 29 Fan

Claims (7)

A first-stage nitriding step of nitriding a steel member in a nitriding gas atmosphere at 550 to 610 ° C. with a nitriding potential at which ε-phase or ε-phase and γ'phase nitriding compound layers are formed.
Following the first-stage nitriding treatment step, there is a second-stage nitriding treatment step in which the nitriding-treated steel member is held at 480 to 520 ° C. for 10 to 60 minutes in a nitriding gas atmosphere. Nitriding treatment method for steel members. The method for nitriding a steel member according to claim 1, wherein the second-stage nitriding process is performed in a nitriding gas atmosphere having a nitriding potential of 0.2 to 2.0 atm ^−0.5 . The method for nitriding a steel member according to claim 1 or 2, wherein the nitriding potential in the first-stage nitriding process is 0.2 to 2.0 atm ^−0.5 . The method for nitriding a steel member according to any one of claims 1 to 3, further comprising a cooling step of cooling the steel member to room temperature after performing the second-stage nitriding treatment step. The method for nitriding a steel member according to any one of claims 1 to 4, wherein in the second-stage nitriding process, the nitrided steel member is held at 480 to 520 ° C. for 10 to 45 minutes. The method for nitriding a steel member according to any one of claims 1 to 4, wherein in the second-stage nitriding process, the nitrided steel member is held at 490 to 510 ° C. for 10 to 45 minutes. A first-stage nitriding step of nitriding a steel member in a nitriding gas atmosphere at 550 to 610 ° C. with a nitriding potential at which ε-phase or ε-phase and γ'phase nitriding compound layers are formed.
Following the first-stage nitriding treatment step, a second-stage nitriding treatment step is performed in which the nitrided steel member is nitrided under the condition that the γ'phase fraction of the nitrided compound layer is increased by 30% or more within 60 minutes. A method for nitriding a steel member.